Google App Script: GAS return 403 but browser requests still work

I’m using google app script to monitoring URL is working then writing log in excel after every 30 mins:

  try {
    var options = {
      "method" : "GET",
      'headers': {
        'User-Agent': 'Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/96.0.4664.93 Safari/537.36'
      },
      'muteHttpExceptions': true
    };
    var url_trimmed = url.trim();
    var response = UrlFetchApp.fetch(url_trimmed, options);
    Logger.log(response.getContentText()); 
    var result = response.getResponseCode();
    return result;
  }
  catch (err) {
    return [EXCEPT_ERR_CD, Utilities.formatString('statusCode：　%s、Error: %s）', EXCEPT_ERR_CD, err)];
  }
}

But when I check Excel result file, result sometimes return responseCode 200, and sometimes return 403. Immediately when responseCode returns 403, I check it in browser but site is working fine.
I tried add header parameter but it doesn’t work.
I dont have any idea, I think maybe the error from the site but dont really know causes and how to fix it.
Thank you very much.